Programmable controllers are commonly used in modern factories to control various industrial processes.
The programmable controller, or PC, functions as a replacement for electromechanical relays and coils and minicomputers.
The programmable controller reads the present value of an input, makes a logic decision based on the input value in accordance with a stored program and generates an output signal to drive an output device to either an "On" or "Off" state.
Input values include feedback signals representing parameters of the controlled process, output signals generated by other PCs or operator actuated devices such as pushbutton switches, or pushbuttons, which allow the operator to enter information or commands into the PC.
For each operator command entered into the PC, a specific pushbutton is required, along with the resulting cabinet space and hard-wired connections. In a large industrial process this requires very large cabinets containing many hard-wired connections. Industrial grade switches and the labor required to install same can become quite expensive and as is commonly realized in the industry, each additional device or connection thereto corresponds to an additional source of component and, hence, system failure.
As the quantity of operator pushbuttons increase, the operator is progressively given a more error prone task of selecting the correct pushbutton with which to respond.
In addition, as the industrial process is later modified and additional operator activated devices are required, additional pushbuttons must be installed.
In the event of an alarm condition requiring an operator response, the output signals may actuate a general alarm indicator such as a lamp, or they may actuate one of a plurality of indicator lamps corresponding to the specific problem or the specific operator response required.
Such alarm indications do not inform the operator of the sequence in which the alarms occurred, of the priority of the alarms nor of the proper response thereto.
In addition, the operator upon observing the alarm condition, must first determine and then locate the correct input pushbutton with which to respond.